You can unlock additional heroes by obtaining the hero cards. The first legendary (hero) card you find unlocks the hero for you to use and upgrade to up to level 33. The next card of the same hero lets you upgrade them up to the level 66 and 3rd card of the same hero lets you upgrade it up to level 99.
You can find these cards in daily, golden and epic chests. Alternatively, if you want to ensure that you get a hero card, you can buy the new special hero chest available in the ingame shop after you reach rank 17 on your account, which is, however, available for purchase only once.
